This invention relates generally to image registration boards and compressible pins of the type used in photomechanical platemaking primarily for lithographic printing, and particularly to those used in exposures utilizing vacuum frame or step-and-repeat registration systems for making plates, proofs or contact prints, and the like.
Most printing processes involve the step of making an intermediate image carrier which transfers the ink to the substrate (paper, fabric, film, etc.) to form a predetermined printed image on that substrate. The lithographic plate is normally a smooth, flexible sheet with the image and non-image areas confined within the same plane. This may be contrasted with gravure or letterpress printing, in which the image carrier has a rough surface with raised areas corresponding to the shape of the printed image.
In most photomechanical lithographic printing, the plate has an emulsion layer of one or more light-sensitive coatings and a metal backing. A negative image (or "flat") is placed directly on top of the plate, and the particular image corresponding to the clear areas of that negative is exposed or "burned" onto the plate using high intensity light. The area of the emulsion exposed to the light fuses to the metal backing and becomes chemically receptive to oil-based inks. The exposed plate is developed by wiping a developing chemical on the emulsion layer to remove the unexposed light-sensitive coatings, thereby leaving a smooth but visible image behind.
The developed plate is usually mounted on a rotating cylinder and coated with a thin layer of water during each revolution, with the water being attracted to the unexposed or non-image areas of the plate. Ink is applied to the plate during each revolution, with the ink being repelled by the water and attracted to the exposed or image areas of the plate. The ink is then transferred from the plate to the substrate as the cylinder and plate rotate. In offset lithographic printing, the ink is transferred from the image carrier to an intermediate blanket cylinder, and subsequently to the substrate.
The plates are generally burned within a machine called a "platemaker," which may be manual, automated, or a combination thereof. Most commercial photomechanical platemakers now utilize either a vacuum frame or a step-and-repeat machine.
In most platemakers, a fresh plate is placed emulsion side up on a supporting blanket or surface, and one or more flats are placed overlying the plate. A glass cover is lowered onto the flats and plates to press them into contact with one another and the underlying blanket. A vacuum is drawn between the blanket and glass cover, which pulls the flats into tight and uniform parallel contact with the plate. The light is then passed through the glass cover and flat to expose the plate.
Most platemakers use a pin register system to ensure proper and reproducible alignment between flats and plates, both before and during burning. The system comprises a row of small metal posts or registration pins that correspond to registration holes punched in the edges of both flats and plates. The flats and plates will be in correct alignment each time the punched registration holes are placed over pins having the corresponding registration positioning or array.
Prior to the actual platemaking exposure, the necessary flats must also be prepared by a process termed "page makeup" or "stripping." This involves positioning and adhering one or more negatives on a masking paper or polyester film, which then becomes the flat. The negatives are taped to the masking material, and the masking covering the image area is cut away to produce a window through which the light may pass. Light will be blocked by either the masking or the image area of the negative.
One flat may include all the negatives required for a specific plate, or a lesser number. For example, separate flats might be prepared for the halftone negatives and solid type to be printed in one color from a single plate. Making one plate from one flat uses one exposure or a "single burn." If two flats are used to make one plate, two exposures or a "double burn" are required. Additional flats for corrections, tints, and reverses may also be made in separate steps, and three or more exposures may then be used to make one plate. The multiple exposure of a single plate using more than one flat is generally referred to as "surprinting." Screen tints and some negative halftones may be accomplished by placing a ruled or dotted line screen between the particular flat and the plate.
The layout of negatives into flats and the alignment of flats for a plate may be done on a light table or layout device and then transferred to the platemaker. In some cases, page makeup may be performed on the platemaker itself. The page makeup process may take one step, or be extremely complicated. Consequently, proper registration or alignment of the various negatives, flats, and plates must be maintained during layout, stripping, transfer, as well as within the platemaker. Registration must also be consistent between plates used for each different type or color of ink in a multi-impression page, or when multiple plates are required for large runs.
Copy will generally be adhered to a mounting board and covered with an acetate overlay. Color breaks and other guide information will be written on other tissue overlays. The stripper will use register marks, corner marks, and keylines to position the negatives relative to the copy on the mounting board, and may also make reference to or actually use dummies and the original mechanicals to make negatives. A base negative generally shows as much copy as possible on a mounting board. The stripper may mask the original mechanical to prepare multiple negatives, or mask the base negative to make multiple plates for different colors. Smaller flats (from the same or different jobs) may be ganged together to make multiple plates in the same exposure on a single sheet of film or plate material. As an example, a sixteen page book printed in two colors with ten halftones requires forty-two negatives (sixteen for each color and ten for the halftones) assembled into six flats. Three flats are used for each side of the paper, comprising one set of type and line art for the first color, one set of type and line art for the second color, and one set for halftones in the first color. A larger book may have one thousand negatives assembled into one hundred flats, all which require uniform registration during layout, stripping, transfer, and platemaking.
Vacuum frames are generally used if all the negatives are stripped on a single flat or if one flat is used for two or more exposures on a plate. In that case, register pins are used to make sure the exposures are made in proper alignment. In contrast, if plates are made from positives, all the elements must be combined into a single flat before exposure.
Some vacuum frames permit variable exposure settings or utilize computerized programs for making plates, as well as proofs or contact prints (prints frequently used as test sheets made by exposing a negative in direct contact with photographic paper, and sometimes used as final output.) Some vacuum frames are adapted for high speed vacuum drawdowns to accelerate the exposure process. In any case, it is important to draw a strong and uniform vacuum in the vacuum frame, in order to maintain close uniform contact between the flats and plates. Close contact between the flat and plate prevents dot gain by light spreading between a negative flat and plate, or dot sharpening if the flat is a positive.
When a plate requires more than four exposures, or particularly accurate alignment, the vacuum frame is less economical and accurate than a step-and-repeat machine. The conventional horizontal step-and-repeat machine has a bed to support and mount the plate, with the flats and plate being held in tight contact by a vacuum during burning. The flats are mounted in a predetermined spaced-apart array on a chase which may be moved in exact increments transversely across the plate in two directions. A stationary lamp positioned above the plate makes each exposure, with the chase being stepped across the plate from one flat to another between exposures, and the process repeated for the number of flats and exposures required. The movement of the chase corresponds exactly to the array of flats.
Some step-and-repeat machines move the chase in response to commands from a program, and may be automated using cassettes containing films which correspond to each flat. Automated step-and-repeat machines are particularly useful where many subjects are burned on the same plates using a multiplicity of sequential flats, or in book production and computer-based publications where the layout and separation of each page is done on computer and printed directly to film negatives.
The flats may be positioned and repositioned on the chase or manual step-and-repeat registration board in larger spacing increments (usually of about one half inch) corresponding to the distance between movable pins placed in indexing holes in the chase or board, and may be positioned or shifted between the spacing increments in smaller increments using spacers. An example of this technique is disclosed in U.S. Pat. No. 2,983,049 to Andrisani.
The stripper can therefore perform the necessary page makeup by assembling the negatives into flats and positioning the flats on a step-and-repeat registration board, with the desired registration format being easily reproducible in subsequent layouts or on another registration board containing other elements of a plate. Shifts or corrections in the format may be translated to each registration board. The flats may also be transferred to any suitable chase in the desired format.
The process for making an intermediate image carrier should be contrasted with quick printing, in which a camera-direct paper or plastic plate (usually termed a "master") is made directly from the camera-ready artwork or "mechanical" using a process camera. Masters are generally good for less detailed images in which the run will be fewer than five thousand impressions, and masters do not require the more labor intensive steps of making and stripping negatives. Quick printing may be suitable for producing "good" quality work. However, photomechanical production of plates is needed when greater accuracy or reproducibility is required, or when the output is to be "premium" or "showcase" quality.
As such, improvements designed to minimize, accelerate, or automate the labor intensive steps of plate production are of great value to the industry. Of comparable value are improvements which insure or enhance output quality, particularly when those improvements are combined with systems to accelerate or automate any steps in the plate production process.
In a vacuum frame, the pivoting cover may press the register pins down into the blanket at an angle and tilt the posts, thereby causing the flat and plate to shift or buckle relative to one another. Alternately, discontinuities in the resilience of the blanket may lead to tilting of the register pins. This may lead to misalignment between the flats and plate and other distortions which may be carried through to or compounded by subsequent burns. Increased numbers of negatives and masking layers in a flat or combined flats and screens will lead to greater misalignment and distortion. Localized irregularities around the register pins (especially adjacent to the edges where the vacuum is drawn) may lead to a non-uniform vacuum being drawn over portions of the plate, thereby resulting in blooming or sharpening over larger areas of an image.
It is known to utilize register pins having smaller diameter bases than conventional metal base pins to mitigate against tilting, buckling, and distortion. It is also known to utilize register pins having compressible or retractable posts, or register pins which are mounted on a cushioning material within the board that deforms under pressure before the blanket.
However, such register pin assemblies incorporating compressible or retractable posts present a greater overall height than can be accomplished using a solid register pin. Since the body of the register pin must be disposed flush with or beneath the upper surface of the image registration board, this increased height requires a thicker image registration board to receive the register pin assembly. Alternately, a portion of the registration pin assembly must project beneath the base layer of the image registration board, thus deforming the blanket or permitting the register pins to be dislodged when the image registration board is placed on any rigid working surface.
In addition, it is desirable to minimize the thickness of the image registration board in order to decrease the weight of larger boards, reduce the displacement between the cover and the blanket in a platemaker, and to some extent control or compensate for the effect of differential thermal expansion between layers within the image registration board.